Photoresponsive electrostatic image recording apparatus with charging electrode matrix array



NOV. 5, 1968 Q CQWAN ET AL 3,409,899

PHOTORESPONSIVE ELECTROSTATIC IMAGE RECORDING APPARATUS WITH CHARGING ELECTRODE MATRIX ARRAY Filed Sept. 1, 1964 5 Sheets-Sheet 1 FIG' I CARL E- COWAN STEPHEN MICHEL INVENTORS ATTORNEY AND AGENT Nov. 5, 1968 c. E. COWAN ET AL. 3,409,899

PHOTORESPONSIVE ELECTROSTATIC IMAGE RECORDING APPARATUS WITH CHARGING ELECTRODE MATRIX ARRAY Filed Sept. 1, 1964 5 Sheets-Sheet 2 FIG- 9 0 E E E E Z CARL E- COWAN STEPHEN MICHEL INVENTORS BY 77 I04 ATTORNEY AND AGENT Nov. 5, 1968 c. E. COWAN ET AL 3,409,899

PHOTORESPONSIVE ELECTROSTATIC IMAGE] RECORDING APPARATUS WITH CHARGING ELECTRODE MATRIX ARRAY Filed Sept. 1, 1964 5 Sheets-Sheet 5 CONDUCTIVE BACKING MEMBER 101 l INSULATING MATERIAL 102 CHARGING /j ELECTRODES n5 5 SELENIUM 3. 1

LAYER J2 in L '\'t\ j A 1 I J V l I, f i i\ OPAOUE MASK I05 I00 t GLASS I03 INTERDIGITATED CONDUCTORS 107,109

FIG- I2 OF VOLTAGE ACROSS ARRAY OF ELECTRODES RESISTANCE OF PHOTORESPONSIVE ELEMENT CARL E' COWAN STEPHEN MICHEL INVENTORS ATTORNEY AND AGENT United States Patent 51cc i 3,409,899 PHOTORESPONSIVE ELECTROSTATIC IMAGE RE- CORDING APPARATUS WITH CHARGING ELEC- TRODE MATRIX ARRAY Carl E. 'Cowan and Stephen Michel, Rochester, NY. assignors to Eastman Kodak Company, Rochester, NY,

a corporation of New Jersey Filed Sept. 1, 1964, Ser. No. 393,641 3 Claims. (Cl. 346-74) 9 ABSTRACT OF THE DISCLOSURE A light-image to charge-image transducer having an interdigitated conductor array with transversely overlying photoresponsive strips, each strip having-alternately masked and unmasked portions with a charging electrode at each junction of the masked andunmasked portions forming an electrode matrix array. A potential is applied between adjacent conductors and in response to a light image the photoresponsive strip acts as a voltage divider with the potential on selective electrodes charging to simultaneously efiect an image charge pattern on a dielectric record medium inserted between the electrode matrix array and a backing electrode.

The present invention relates to an apparatus and method for generating an electrostatic image for simultaneous or later development and more particularly to an apparatus and method for producing distribution of electrostatic charges on an insulating material to form such an image. Xerography is a well known electrostatic recording system in which a surface is uniformly charged and then an image is formed by removing some areas of the charge. The present invention eliminates the advance uniform charging step and forms the electrostatic image by selectively applying the charge. Xerography also requires use of a photoconductive layer which either is used up in connection with a single image or must be cleaned with great care and then recharged before reuse. In practicing the present invention none of these inconveniences occur; it permits multiple reuse of a photoconductive element without cleaning.

In the present invention the many disadvantages of the prior art are not only overcome, but additional advantages are obtained in that either a negative or a positive image can be produced. One embodiment of the invention illustrated in the apparatus described hereinbelow, uses a voltage source at least across a portion of which a voltage divider circuit comprising a photoresponsive element and a resistor is connected. A change in a controlling radiation, for example, from dark to light, that is incident on the photoresponsive element produces a change in resistance, which causes a change in voltage at the junction point of the photoresponsive element and the resistor. This fluctuating voltage is transmitted to an insulating recording medium by a charging electrode. With such a system, a negative or positive copy can be made merely by interchanging the photoresponsive element and the resistor and does not necessitate any change in the medium used as a developer. For either positive or negative images, the sensitivity or threshold operating level of the circuit can be altered by changing the supply voltage, the illumination level for an image of given contrast, the load resistance, and/or the material from which the photoresponsive element and/ or the load resistance is made.

A primary object of the invention is to provide a new and improved apparatus and method for converting a controlling radiation having predetermined characteris- 3,409,899 Patented Nov. 5, 1968 tics into an electrostatic image that can be recorded on an electrically insulating material.

Another object of the invention is to selectively electrostatically charge an insulating material in dependence upon a controlling radiation having continuously changing characteristics, or upon a controlling radiation having predetermined characteristics, the charged image on said material being of either the same or a different size than the controlling radiation.

Still another object of the invention is to provide apparatus in which a controlling radiation having predetermined characteristics is displayed on an array of photoresponsive elements to control the voltage across an insulating material at its respective charging electrode for selectively charging said material.

A further object of the invention is to provide an apparatus for electrostatically recording a controlling radiation having predetermined characteristics in which an individual voltage divider circuit is connected to each charging electrode in an array for controlling the voltage across an insulating material in accordance with the absence or presence of said radiation incident on a photoresponsive element forming a part of each such circuit.

Another object of the invention is to provide an apparatus for selectively electrostatically charging an insulating material in dependence upon controlling radiation in which either a negative or a positive copy can be produced.

A further object of the invention is to provide an apparatus for selectively electrostatically charging an insulating material in which a wrong-reading copy of an original can be produced which serves directly as a printing master, the wrong-reading copy being produced by reversing either the orientation of the optical image or the direction of movement of the material relative to the original.

And a still further object of the invention is to provide an electrostatic reconding head that comprises a single unit including a voltage divider circuit that is associated with each of the charging electrodes.

The present invention relates to a device for depositing a charge by means of a stylus or electrode which point-scans an insulating material, or a row of such electrodes which line-scans such a material, or an array of the equivalent of such electrodes which deposits a complete charge image on an area corresponding to that of the array. By working with electrodes that are approximately uniformly spaced from a conductive backing member, such spacing not being critical, the row of electrodes for line scanning, according to one embodiment of the invention, produces substantially uniform charges across the full width of the insulating material. This spacing of the electrodes from the material is chosen such that it is at the low point of the Paschen curve (breakdown voltage vs. the product of pressure and distance) where, as known by those skilled in the art, a small variation in such spacing does not affect the sparking potential significantly.

In one embodiment of the invention, a stylus or electrode is maintained at a potential by being connected to a point between a resistance and a photoresponsive element that are connected, in series, across at least a portion of a voltage source of potential. The electrode is spaced from a conductive backing member by a distance slightly greater than the thickness of an insulating material on which the charge is to be deposited. When the electrode is at high potential, a corona is formed and deposits a charge on the insulating material which is positioned between the electrode and the conductive member and is in contact with the latter. In one embodiment of the invention the photoresponsive element is connected between the electrode and ground so that when light is incident on the photoresponsive element, the voltage at the electrode falls to a value at which the corona is extinguished. In a second embodiment, the photoresponsive element is connected to the high side of the voltage source or otherwise, so that light falling ont hat element raises the electrode potential from'a value below corona to a value which causes corona. In both embodiments, the potential on the electrode is effectively independent of the resistance between the electrode and. the backing member.

Other objects and advantages of the invention will be apparent to those skilled in the art from the description which follows when read in conjunction with the accompanying drawings.

Reference is now made to the accompanying drawings wherein like reference numerals designate like parts and wherein:

FIG. 1 is a schematic wiring diagram showing a photoresponsive element connected in series circuit with a charging electrode as used in the present invention;

FIG. 2 is a schematic wiring diagram showing another embodiment of the circuit as used in the present invention;

FIG. 3 is a schematic wiring diagram showing another embodiment of the circuit shown in FIG. 2 for switching the elements of a voltage divider circuit for producing a negative or positive electrostatic image on an insulating material;

FIGS. 4 and 5 are vertical sectional views of a recording head as used in FIGS. 7 and 8, respectively;

FIG. 6 is a partial plan view of the recording head shown in FIGS. 4 and 7;

FIG. 7 is a schematic representation of a copying apparatus according to the invention wherein a controlling radiation is projected onto a recording head;

FIG. 8 is a schematic representation of another embodirnent of the invention in which a recording head is arranged between an original and an insulating material for providing an image of unit magnification;

FIG. 9 is a plan view of a recording head comprising an array of closely grouped charging electrodes;

FIG. 10 is a sectional view taken substantially on the line 10-10 of FIG. 9 and showing the relation of the recording head to an insulating material and a conductive backing member;

FIG. 11 is a perspective view of a' portion of the elements shown in FIGS. 9 and 10;

FIG. 12 is a schematic wiring diagram disclosing an equivalent circuit produced by the recording head shown in FIGS. 9 and 10; and

FIG. 13 is a graph of a plot of photoconductor resistance versus the percent of the voltage across an array at the charging electrodes for the recording heads shown in FIGS. 4, 5, 9 and 12.

One system of recording, according to the prior art, involves a photoresponsive element and a charging electrode in series circuit, the electrode being spaced from an insulating material on which the charge is to be recorded, the latter being positioned in contact with a conductive backing member that is connected to ground. A source of potential has one side connected to ground and the other side connected to the photoresponsive element. When radiation is incident on the photoresponsive element, its impedance is lowered to a point where substantially the entire supply voltage appears between the electrode and ground, so that a corona is formed which deposits the desired charge on the material. However, even when the photoresponsive element is dark, it still has a finite (through very high) resistance and therefore does not completely break the connection between the source and the electrode. As a result, the potential on the electrode, even if originally below a coronaforrning value, will rise at a rate at least partially dependent upon the stray capacity inherently present,

.4 until it reaches a value sufiicient to start a corona discharge. This, in turn, will quickly discharge the capacity until the electrode potential falls to a level Where the corona will be once again extinguished. The changing and discharging cycle will repeat over and over, each discharge resulting in the deposition of a corresponding charge on the material, the over-all result being a smudging and loss of contrast in the final copy.

The present invention overcomes this disadvantage in that the circuit is modified by the addition of a fixed resistance connected in series circuit with the photoresponsive element across at least a portion of the source of potential. This circuit is shown in FIG. 1 wherein an insulating material 10 is positioned between an end of electrode. 11 and a conductive backing member 12, the material 10 being in contact with the member 12. A photoresponsive element 13 and a fixed resistance 14 are connected in series circuit and across a source of electrical potential 15 that is greater than that necessary to produce -a corona discharge between the electrode and the material. The electrode 11 is connected to the series circuit between the photoresponsive element 13 and the resistance 14. The backing member 12 is connected to one terminal of the battery 15 which, as shown, is the ground side.

The resistance 14 is a fixed resistance and is substantially equal to that of the photoresponsive element 13 when the latter is in the dark. These elements form, in effect, a voltage divider circuit and in a practical application such a circuit is connected as described to each of a plurality of the electrodes 11. With the voltage from battery 15 of the polarity shown, the changing electrode 11 is at a certain voltage negative with respect to ground when the impedance of the photoresponsive element 13 is high. When the photoresponsive element is energized, its impedance drops and the voltage at thecharging electrode becomes less negative than it was before. If voltages are chosen such that with the photoresponsive element dark, the charging electrode is sufliciently negative to put a charge on the insulating material, and with the photoresponsive element energized, the voltage is not sufiiciently negative to charge the material, an imagewise change will be applied on the sheet as scanning or projection of an original is efiected. However, with a circuit such as shown in FIG. 1, it has been found that the voltage necessary for producing an electrostatic charge presents a problem in that excessive heating of the photoresponsive element tends to occur with a sufficiently high voltage.

Since the voltage diflferences obtainable across the photoresponsive element 13 depend on the voltage of the battery 15 and since breakdown potential of an array of electrodes is limited, a convenient way to obtain voltages that are sufliciently high to charge the insulating material and at the same time eliminate the heating problem is to include a bias voltage as shown in FIGS. 2 and 3. For minimum heating of the photosensitive element, the bias voltage should be as high as possible without raising the electrode potential to a level that will sustain corona discharge when the photosensitive element is energized. As a practical matter, a voltage of the same general magnitude as the corona extinction voltage is appropriate.

In FIG. 2 the primary voltage and the bias voltage are derived from a voltage divider circuit comprising resistors 16 and 17 that are connected across a source of electrical potential 18, the voltage across resistor 16 controlling the primary voltage and that across resistor 17, the bias voltage. In this circuit the fixed resistance 14 and the photoresponsive element 13 are connected in series circuit and across the resistor 16.

In FIG. 3 a source of electrical potential comprises a battery 19 providing a primary source and a battery 20 providing a bias source. As also shown in FIG. 3, the

fixed resistor 14 of, FIGS. 1 and 2 can be replaced by an adjustable resistor or by, another photoresponsive element 21 whose impedance can be controlled by an auxiliary light source 22. With this arrangement the impedance of the adjustable resistor or of the photoresponsive element can be adjusted to matchthe impedance of a photoresponsive element 23 [for any level of energization of the latter.

With either a linear or an area array of photoresponsive elements, a negative or a positive copy can be produced without changing the developer. This can be accomplished by interchanging the photoresponsive element with the resistor in the circuit as by means of a double-pole, double-throw switch 24 as shown in FIG. 3. By means of such. a, switch the polarity of the source of potential need not be changed.-

As is well known, photoresponsive materials, such as cadmium sulfide, have a. predetermined impedance in the dark and a lesser impedance when subjected to radiation. By radiation is meant .not only visible radiation, but also non-visible radiation inasmuch asphotoresponsive materials are known that provide a different impedance when dark than when subjected to such non-visible radiation. For this reason, applicants do not wish to be limited in their disclosure to the use of only visible radiation to produce or generate electrostatic charges in the manner to be disclosed hereinafter.

A recording head embodying the circuitrydescribed above can take a form as shown, greatly enlarged in FIGS. 4 and of the drawings. The primary difference between a head 27 (FIG. 4) and a head 28 (FIG. 5) is the position of the photoresponsive material or element on which a controlling radiation of predetermined characteristics is incident. Each head comprises a dielectric plate 30 which can be made of a material. such as an epoxy laminate. Adjacent one end thereof, as shown in FIG. 4, the plate 30 is provided with a recess 31 ,in which a; glass support member 32 is arranged so that the exposed surface thereof is fiush with the surface of the plate. A plurality of conductors 33 are arranged inspaced, parallel relationship on the surface 34 of the plate, see FIG. 6. A photoresponsive element 35, such as cadmium sulfide, i positioned on the glass support member 32 in the form of strips so that each photoresponsive strip. is in alignment with a corresponding one of the conductors 33. A connector element 36 overlies each photoresponsive element and its respective conductonA similar connector element 37 overlies its respective element and a bus bar 38 that interconnects all of the connector elements 37. The areas of the photoresponsive element 35 that are not covered by the connector elements 36 and 37provide a linear array of photoresponsive elements on which a controlling radiation of predetermined characteristics is incident as described hereinafter. Intermediate the ends of conductors 33, an insulating member 39 is arranged over each of the conductors. An electrically resistive material 40 forms a fixed resistance that is arranged on each of the insulating members 39 and is joined by a connector 41 to its-respective conductor. A bus bar 42 interconnects all of the resistors 40.

I The recording head 28 shown in FIG. 5. is substantially the same as that shown in FIG. 4 with the exception that a glass strip 45 is mounted on an end plate 30 and a plurality of photoresponsive elements 46 are arranged on the facing end of the strip 45. Each of the conductors 33 extends to the ends of the photoresponsive elements 46 and the bus bar 38 is secured to the other surface 47 of plate 30 and in contact with the photoresponsive elements 46. This embodiment of the recording head is particularly adapted for apparatus, such as shown in FIG. 8, wherein a document is continuously scanned and an image of unit magnification is produced.

The recording heads 27 and 28 can be constructed by using photoresist techniques and both etching and sandblasting procedures to obtain the desired conductor pattern described aboye. While actual dimensions can be varied depending on the results desired, excellent results have been obtained using a recording head with 125 conductors per inch wherein the active length of each photoresponsive element, that is, the length exposed to the controlling radiation, is approximately 0.005 inch. In such recording heads, the conductors 33 provide a linear array of closely grouped elements and can be used for copying with either a reflection or transmission system, as described hereinafter.

With reference to FIGS. 4 and 5, a source of electrical potential greater than the necessary to produce acorona discharge comprises a battery 50 connected across the bus bar 38 and the bus bar 42 and a battery 51 connected between ground and the bus bar 38. Each resistor 40 and its respective photoresponsive element are in series and therefore form respective legs of a voltage divider circuit connected across a portion of the voltage source. At the junction 52 of each circuit, see FIGS. 2, 4 and 5, the voltage is brought out by conductor 33 which forms an electrode designated by 53 that is spaced from a conductive backing member 54 by a distance greater than the thickness of the insulating material 10 which is in contact with said member 54. It has been found that with a voltage across the divider circuit of 200 volts, a bias voltage of 300 volts. the photoresponsive elements 35 in the dark, and the fixed resistances equal to the dark resistance of the photoresponsive elements, the voltage at the charging electrodes 53 is -400 volts. With meter candles of radiation incident on some of the photoresponsive elements 35, the voltage at the corresponding charging electrodes drops to 325 volts. A scanning rate of 0.5 inch per second which is 0.1 second exposure per 0.005 1 inch of length of photoresponsive material provides an exposure which is equivalent to that of an ASA 30 high contrast material. An insulating material, such as paper coated on one side with a thin layer of an insulating polymer, can be moved relative to or positioned on the backing member 54 with the ends of electrodes 53 spaced approximately one mil from the insulating layer on the paper. This latter dimension can be maintained very accurately by providing a layer 55 of a material, such as Teflon, on the edge of plate 30 adjacent electrodes 53. A voltage swing of about -100 volts has been found to be adequate to start and stop the discharge, .the exact voltage swing depending on factors such as the particular insulating material used, the type and concentration of toner to be used, the magnitude of the distance between electrodes 53 and the material 10, the shape of the ends of the electrodes 53, etc. In the example given above, it has been found that the ends of electrodes 53 can vary in spacing from the insulating layer on the paper by one mil, that is, the spacing can be one-half mil to one and one-half mils without necessitating any change in the voltage of potential. 7

The recording head 27 can be made by starting with a filmof copper laminated to an epoxy glass substrate designated by the numeral 30 in FIG. 4 which is then coated with a resist material and exposed to a light negative to give the desired patten for conductors 33. The unprotected copper film is etched with ferric chloride and one end of the array is left unetched to become the bus bar 38. The recess 21 is milled in the plate 30 adjacent the bus bar 38 and is of sufficient depth to take the glass strip 32 that is coated with sintered cadium sulfide so that the top surface of the photoresponsive material is flush with the top of the conductors 33. A portion of the array is then covered with a thin layer of an epoxy resin to provide the insulating member 39 and a coating of resistance material 40 consisting of colloidal graphite in epoxy resin is applied on the top of the insulating material 39. The resistance material 40 is coated with a thick layer of resist material that is exposed to the same negative used for the conductors and positioned in registry with the latter. The resistance material that is not protected by the resist material is sandblasted away leaving thin strips of resistance material in registry with the conductors. One edge of the two layers comprising insulating members 39 and resistance material 40 is sandblasted at an angle to give a gradual slope thereto as shown in FIGS. 4 and 5. Individual connectors 41 join each resistor so formed to its respective electrode 33 at one end and can be evaporated through a mask placed in registry therewith. The bus bar 42 is arranged across the array by an evaporation process and shorts all of the resistances 40 at theother ends thereof.

' The cadmium sulfide 35 on the glass strip 32 is cut into individual photoresponsive elements by sandblasting though a metal mask. The glass strip is then cemented in the recess 31 and individual connectors are then evaporated connecting one end of each photoresponsive element to a conductor 33 and the other endof each photoresponsiveelement to the bus bar 38. The bus bars 38 and 42 are the connection points :for the circuit as described above. The procedure described above can be used in a modified form to produce the array as shown in FIG. 5. The glass strip 32 on which the cadmium sulfide is sintered must 'be either black or at least blackened on the back side to reduce light scatter within the glass. Evaporative methods for making the resistors 40 and the photoresponsive elements 35 can also be used.

A particularly valuable advantage of the invention is the sensitivity control or threshold operating point that can be obtained. This is shown in FIG. 13 which is a plot of resistance of the photoresponsive element versus the percent of the voltage across the array appearing at the charging conductors. With the dark resistance or impedance of the photoresponsive element equal to the fixed resistance, point A, 50% of the voltage across the array appears at the charging conductors. This voltage is added to the bias volt-age to obtain the voltage of the charging conductor to ground. Energization of the photoresponsive element to lower its impedance to point B decreases the percentage of the array voltage to be added to the bias voltage. It can be readily appreciated that the first decade change in impedance accomplishes most of the voltage change that is available. If the dark impedance of the photoresponsive element is picked to be at point C, a one decade decrease in the impedance thereof brings the light operating point to'point D which results in less voltage change at the conductors than in the first example and, hence, less sensitivity. For the optimum sensitivity that would be desirable for light copy work, the operating point should be centered, that is, the dark impedance of the photoresponsive element should be chosen to be at point E. Then with a one decade change in impedance the light operating point would be at point C.

In all of the above discussions it makes no diiference whether the impedance of the photoresponsive element is changed to points A, C, or D, or if the fixed resistance is changed to accomplish the same result. For this reason, the use of a variable resistance, such as described above with respect to resistor 21, is important in that it permits setting the ratio of fixed resistance to dark impedance of the photoresponsive element to any desired value. In addition to the method of changing sensitivity by operating on various parts of the curve shown in FIG. 12, the voltage across the array can be increased or decreased to increase or decrease sensitivity, respectively. With a decreased voltage across the array, less voltage change results with a given change in impedance of the photoresponsive element and results in a decreased sensitivity. The converse is true with an increase in voltage across the array.

The above method of changing sensitivity involves changing the operating parameters of impedance ratios and voltages. The sensitivity can also be changed by using photoresponsive elements of different sensitivity in that a photoresponsive element requiring more light for a given change in impedance gives a lower sensitivity. All of these variations are possible because the change in voltage at the electrodes required to give the minimum to maximum density range is fixed forgiven conditions of toner, copy medium and electrode gap. It is to be understood that if operating points on the curve are changed, the bias voltage must also be changed to bring the points in the charging voltage range.

With particular reference to FIG. 7 a document 60 is moved continuously past a slit 61 in a plate 62 by spaced endless belts 63 that encircle a set of pulleys 64 and 65, the document being held in engagement with the belts by a plurality of transversely spaced presser rollers 66 and 67. One of the pulleys 64 or 65 can be driven in a direction as shown by an arrow 68 so as to move ,the document 60 past the slit 61. A source of .energization, such as a lamp 69 is arranged below the belts. Successive portions of the document moving past the slit 61 are imaged by means of a projection lens system comprising lenses 70 and 71 and a mirror 72 onto the photoresponsive elements of an electrostatic head generally designated by the numeral 73. A copy medium 74, such as described above, is moved past the charging electrodes of the head 73, such as that shown in FIG. 4, by means of a pair of rollers 75 and 76 and guided in its path by rollers 77, 78 and 79. The head 73 is spaced from a conductive backing member 92 by a distance greater than the thickness of the copy medium which is moved over and in contact with said backing member between the latter and said head. The lens system, which is shown as comprising lenses 70 and 71, can be such as to provide a magnified or reduced image of the portions of the document 60, as well as a right or wrong reading copy in accordance with the copy requirements. If document 60 is moved in :a direction opposite to that of medium 74, a right-reading copy can be produced. On the other hand, if document 60 and medium 74 are moved in the same direction, a wrong-reading copy is produced. In either case, the movement of the document 60 and copy medium 74 must be in synchronism in order to obtain a true reproduction. In this embodiment, the document 60 is, in elfect, scanned as it moves past the slit 11 so that the image projected onto the photoresponsive elements of the head 73 is a continuously changing radiation pattern.

Another embodiment of a copy system is disclosed in FIG. 8 by which a reproduction of unit magnification is obtained. The document 60 is moved past a slit 80 in a member 81 arranged within a rotatable transparent drum 82. A pair of rollers 83 and 84 hold the document against the drum. A source of radiation, such as a lamp85', is arranged within the drum 82 for illuminating successive portions of the document as it moves past the head 86 that is positioned between the drum and the copy medium 74. As in the previously described embodiment, the copy medium is moved past the head 86 in synchronism with the movement of the document by drum 82 and in contact with a conducting backing member 87. The copy medium is withdrawn from a supply roll 88 and moved over a pair of rollers 89 and 90 to a take-up roll 91 which can be driven in any suitable manner. In each of the embodiments described thus far, the sources 69 and 85 can be positioned on the other side of plate 62 and outside of drum 82, respectively, to provide a reflection copying system.

'In each of the above described embodiments the heads 73 and 86 are connected to a source of electrical potential 50, 51 as shown and described above with respect to FIGS. 4 and 5. The controlling radiation is a continuously changing pattern of random light and dark areas dependent on the portion of document 60 that is aligned with slits 61 and 80 at any instant. In each embodiment the controlling radiation is incident on the photoresponsive elements 35 or 46 either by means of the projection system shown in FIG. 7 or directly as shown in FIG. 8. The impedance of those photoresponsive elements on which the light areas are incident changes to alesser impedance so that the voltage across the copy medium 74 at eachcorresponding electrode 53 is such, that a corona discharge cannot occur so that no electrostatic charges are applied to the-medium'by these electrodes. The impedance of those. photoresponsive elements on which no light areas areincident does not change and since the voltage across the medium.74 at each corresponding electrode 53,is such that a corona discharge takes place,electrostaticcharges are applied to the medium at these electrodes. Each linear array of electrostatic charges on medium 74. therefore corresponds-to the random dark areas of the controlling pattern of radiation.

While the invention hasbeen described thus far with respect to-=a moving document and amoving copy medium, the same principle can be used for copying a complete document by anarea array of charging electrodes. The .power requirements for such. a system is smaller because only a pulse of voltage is required for 'exposure.,With reference particularly to FIGS. and 12 of the drawings, a recording head 100 is positioned with its electrodes 115 spaced from aconducting backing member 101 by a distance greater than .the thickness of a copy medium 102, such asthe insulating material already described, that is positioned in contact with the facing surface of the memher 101. The head 100 is arranged on a transparent dielectric material, such as a glass plate 103, and the document can be arranged contiguous to the surface 104 and energized by a source, not shown, or the document image can be projected onto this same surface. One method of making an are-a array of charging electrodes is to use selenium for both the photoresponsive element and the fixedresistor, the selenium being masked against energizationto use its high dark impedance as the fixedresistor.

With reference to FIGS. 9- and 10 a series of opaque, insulating rectangles 105 are arranged on the surface 106 f the glass plate 103. A first set of conductors 107 which extend in spaced, parallel relation from a common connector 108 is positioned on. the surface 106 with the conductors 107 arranged midway between said rectangles 105. A second set of conductors 109 which extend in spaced, parallel relation from a common connector 110 is positioned on surface 106 with each conductor centrally arranged over an aligned group of rectangles 105 so that the conductors 109 are interdigitated with respect to conductors 107. A plurality of strips of photoresponsive material 111, such as selenium, is arranged in spaced, parallel relation over and at right angles to conductors 107 and 109 and in registry with each set of rectangles 105. Inasmuch as the elements described thus far can be :lilms of material that can be evaporated on to the surface 106, it is to be understood that where one element overlaps another, the thickness at each such intersection of the elements is less than where the same element can extend to a lower surface. This is shown in a largely exaggerated scale in the cross sectional view of FIG. 10. Strips 112 of an insulating material are positioned at right angles to and over photoresponsive strips 111 and are centrally arranged with respect to each of the conductors 107 and 109. An additional set of strips 113 of insulating material is arranged at right angles to the strips 112 and overlie the space between adjacent strips 111 to fonm openings 114 that extend to the strips 111. An electrode 115 is arranged over each of the openings 114, each electrode being connected to a junction of an area of photoconductive material 111 that is masked by rectangles 105 and an adjacent area of photoconductive material that is unmasked.

In FIG. 12 an equivalent circuit of the structure shown in FIGS. 9-11 is disclosed. It will be evident that each of conductors 109 is centrally connected to a masked portion of the photoresponsive material and that each. of conductors 107 is centrally connected to an unmasked and radiation-responsive area. At the junction of each masked and unmasked area, an electrode 115 is con nected. When the source of electrical potential 50, 51 is connected across the common connectors 108 and 110, a plurality of voltage divider circuits is produced, each comprising a fixed resistance (masked area) and a variable resistance (unmasked area whose impedance is dependent upon the degree of energization thereof), and having an electrode 115 connected to the junction of said masked and unmasked areas. The operation of such a head structure, when an image of a document is incident on the surface 104.is the same as that described above.

The head can take a form other than that shown in FIG. 9; forexample, the rectangles can be separate resistive elements in the same manner .as the insulating equivalent but whose impedance is actually used in the circuit. This has an advantage in that the ratio of the impedance of the photoresponsive element to the fixed impedance can be adjusted during construction.

The recording head,100 shown in FIGS. 9-11 can be made by evaporation techniques in which the rectangles 105 are first evaporated on the glass substrate or plate member 103. The conductors 107 and 109 are then evaporated with the set 109 overlying the rectangles 105 as shown in-FIG. 9. The strips of photoresponsive material 101 (selenium) are then evaporated perpendicular to the conductors 107 and 109 and registered with the rectangles 105 that were previously applied to the plate member. The conductors 107 and 100 form ohmic contact with the selenium strips. The insulating material 112 is then evaporated in strip form perpendicular to the strips of seleni-um with gaps registered midway between the interdigitated conductors 107 and 109. The second set of insulating strips 113 cover the gaps bet-ween the selenium strips with the net result that a series of narrow openings 114 expose the selenium in the area of the junction of the masked and unmasked portions thereof. The electrodes 115 are then evaporated over the insulating strips 112 and 113 and are connected through the openings 114 to the junction of the masked and unmasked areas of the selenium strips.

With reference to FIG. 8, a useful variation of this system is to. interpose an auxiliary drum between the recording head 86 and the recording medium 74. The auxiliary drum can take the form of a metal cylinder that is grounded and has its peripheral surface coated with a thin, electrically insulating material. The charge image on the auxiliary drum can be developed with a charged ink or developer in the normal way at a station intermediate the head 86 and the medium 74. The image is then transferred to the medium 74 by squeegeeing the latter into contact with and peeling it from the auxiliary drum. The advantages of such an arrangement are that an ordinary paper can be used as the recording medium and that the necessary distance between the ends of the electrode array and the auxiliary drum can be set up much more accurately.

A three color image can be generated by the system described hereinabove. This is accomplished by making three successive scans of an original and necessitates having pansensitive conductors and three filters, one of the latter being used for each scan. Alternatively, three arrays of electrodes having red-, greenand blue-sensitive PhOIZO- responsive elements can be used. After each scan of the original, the charge image is developed with a complementary colored ink. After each development and before the next charge image is applied, any residual charge on the recording medium can be removed by an array of grounded or positively charged wires.

While several forms of apparatus and recording heads have been disclosed and described for generating an electrostatic image from the controlling radiation having predetermined characteristics, such apparatus and recording heads are not to be limited to the structures shown but are to be of a scope as defined by the appended claims.

We claim:

1. Apparatus for producing an electrostatic charge pattern on a sheet of material capable of storing an electrostatic charge corresponding to a complete image pattern of controlling radiation of predetermined characteristics, comprising: 1

a conductive backing member against which said sheet of material is maintained;

a transparent dielectric plate having one surface thereof arranged in generally parallel and spaced relation to the'facing surface of said backing member;

a plurality of spaced, parallel electrical conductors arranged on said one surface of said plate, the same ends of' alternate ones of said conductors being joined to a first common connector and the opposite ends of the remaining conductors being joined to a second common connector;

a plurality of strips of photoresponsive material arranged in spaced and generally parallel relationship on said one surface and in transverse, overlying contact with said conductors;

a plurality of opaque members arranged on said one surface beneath each of said alternate ones of said conductors and centrally aligned with each of said strips for dividing each strip into alternate masked and unmasked areas, the unmasked areas being spaced along and centrally aligned with each of said remaining conductors;

means for insulating said members, strips and conductors and providing an opening substantially aligned with the junction of each masked and unmasked area along each of said strips; and

an electrode spaced from said backing member a distance greater than the thickness of said sheet of material and overlying each of said openings, each electrode being electrically connected through its respective opening to said junction aligned therewith;

each portion of said masked and unmasked areas between adjacent conductors forming a leg of a voltage divider circuit for said electrode connected to said junction thereof when a voltage source of potential is connected across said first and second common connectors, each portion of said unmasked area having an impedance substantially equal to that of each portion of said masked area and a lesser impedance upon energization thereof by said image pattern when incident on the other surface of said p ate;

whereby an electrostatic charge pattern corresponding to said complete image pattern is generated on said sheet of material by the electrodes connected to said voltage divider circuits having the unmasked areas thereof energized by said complete image pattern.

2. Apparatus in accordance with claim 1 wherein said insulating means comprises a first set of spaced strips of a dielectric material, each of which is arranged in contact with and transversely of said strips of photoresponsive material and aligned with a respective one of said conductors, and a second set of spaced strips-of a dielectric material, each of which is arranged transversely of said first set of strips and aligned in overlying relationship with a respective space between adjacent strips of said photoresponsive material, whereby a plurality of openings are formed by said first and second set of strips, each of which is substantially aligned with a respective junction of said masked and unmasked areas.

3. Apparatus in accordance with claim 1 wherein said backing member is grounded and including a second voltage source of potential connected between said second common connector and ground for biasing said electrodes.

References Cited UNITED STATES PATENTS 2,817,765 12/1957 Hayford 346-74 3,121,375 2/1964 Fotland 346-74 3,185,999 5/1965 Stone 346-74 3,215,833 11/1965 McNaney 250-495 3,235,874 2/1966 Boyd 346-74 3,267,555 8/1966 Berger 346-74 BERNARD KONICK, Primary Examiner.

L. J. SCHROEDER, Assistant Examiner. 

